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for the specs of the XLSX file format see: https://jkp-ads.com/articles/excel2007fileformat00.aspx or https://www.data2type.de/xml-xslt-xslfo/spreadsheetml/xlsx-format In order to find the correct sheet file one has to first read-in xl/_rels/workbook.xml.rels and save the relations of Id and Target. The file xl/workbook.xml contains a list of sheet elements where for each element the attribute 'r:id' is set. This attribute can be used to look up for the actual data file. This fixes issue 19757